Multi-wavelength surface emitting laser and method for manufacturing the same

ABSTRACT

A multi-wavelength surface emitting laser for emitting light having different wavelengths includes a lower reflector, an active layer and an upper reflector which are integrally formed above one substrate. The multi-wavelength surface emitting laser is manufactured by forming a first surface emitting laser, partially removing a first upper reflector, a first active layer, and a first lower reflection layer by etching. A protection film is formed on the outer surface of the first surface emitting laser. A second surface emitting laser is formed by removing a second lower reflector, a second active layer, and a second upper reflection layer formed on the protection film by etching. The protection film is removed and first and second upper electrodes are formed on upper surfaces of the first and second upper reflection layers, respectively, and a lower electrode is formed on a bottom surface of the substrate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a surface emitting laser whichcan emit laser beams in a direction perpendicular to a substrate, andmore particularly, to a multi-wavelength surface emitting laser in whichdifferent types of surface emitting lasers emitting laser beams ofdifferent wavelengths are configured on the same substrate, and a methodfor manufacturing the same.

[0003] The present invention is based upon Korean Patent Application No.00-41737, filed Jul. 20, 2000, which is incorporated herein byreference.

[0004] 2. Description of the Related Art

[0005] In general, a surface emitting laser emits a laser beam in adirection normal to the surfaces of deposited semiconductor materiallayers, which is different from an edge emitting laser. Also, since thesurface emitting laser emits a circular beam, unlike the edge emittinglaser, an optical system is not needed to correct the cross-sectionalshape of an emitted beam. Also, the surface emitting laser can bemanufactured in a compact size, and a plurality of surface emittinglasers can be integrated on a single semiconductor substrate, thusenabling a two-dimensional arrangement. As a result, the surfaceemitting laser has a wide range of optical application fields such aselectronic calculators, audio-visual apparatuses, laser printers, laserscanners, medical equipment, and communications apparatuses.

[0006] Referring to FIG. 1, a conventional surface emitting laserincludes a substrate 10, a lower reflector 12, an active layer 14 and anupper reflector 16, which are deposited in order on the substrate 10.

[0007] The substrate 10 is formed of a semiconductor material such asgallium arsenide (GaAs) or indium phosphorous (InP) including apredetermined impurity, for example, an n-type impurity. The lower andupper reflectors 12 and 16 are distributed Bragg reflectors (DBRs)formed by alternately depositing pairs of semiconductor layers havingdifferent refractive indices. That is, the lower reflector 12 is formedon the substrate 10 and is formed of the same type of impurity as thatof the substrate 10, for example, n-type Al_(x)Ga_(1−x)As and AlAs arealternately deposited. The upper reflector 16 is formed of the samesemiconductor material as that of the lower reflector 12 and includes asemiconductor material with the opposite type of charge carriers to thatof the lower reflector 12. For example, p-type Al_(x)Ga_(1−x)As and AlAsare alternately deposited. The active layer 14 has a single ormulti-quantum well structure or a super lattice structure as an area forgenerating a light beam when energy transition is generated due torecombination of electrons and holes. An upper electrode 23 having awindow 23 a is provided on the upper surface of the upper reflector 16and a lower electrode 21 is provided on the bottom surface of thesubstrate 10.

[0008] Also, to improve the output of light L emitted through the window23 a, a high resistance portion 18 is formed by injecting protons intothe upper reflector 16 except for a zone under the window 23 a.Accordingly, the high resistance portion 18 limits the flow of holes sothat laser oscillation is generated only in the zone under the window 23a.

[0009] In the surface emitting laser having the above structure, thewavelength of a laser beam is determined by the materials of the upperand lower reflectors 16 and 12, the structure of deposition, and thestructure of the active layer 14. Thus, when the surface emitting laseris manufactured in a single process with a single substrate, it isdifficult to produce the surface emitting laser in an array form so asto emit laser beams of different wavelengths.

[0010] Also, in application fields of a semiconductor laser capable ofemitting laser beams having different wavelengths, such as compatibleoptical pickups for DVD players which can also read CDs, andcommunications equipment in which the wavelengths for transmitting andreceiving are different, a structure in which surface emitting lasers,which are manufactured to correspond to desired wavelengths, and areattached to an additional substrate, has been suggested. However, theabove structure has a problem in that it generates a large opticalalignment error due to an error in the combination of the respectivesurface emitting lasers.

SUMMARY OF THE INVENTION

[0011] To solve the above problems, it is an object of the presentinvention to provide a multi-wavelength surface emitting laser, andmethod of manufacturing the same, formed directly on a single substrateso that optical alignment errors are reduced and manufacturing is madeeasy.

[0012] Accordingly, to achieve the above object, there is provided amulti-wavelength surface emitting laser for emitting light having afirst wavelength and light having a second wavelength. Themulti-wavelength laser comprises a substrate, a first surface emittinglaser for emitting light having a first wavelength, which is directlyformed on a portion of an upper surface of the substrate. The firstsurface emitting laser includes a first lower reflector formed byalternately depositing two semiconductor material layers with the sametype of impurities, but different refractive indices. A first activelayer is provided. A first upper reflector is formed by depositing twosemiconductor material layers having different refractive indices andthe opposite type of impurities to that of the first lower reflector.Also provided is a second surface emitting laser for emitting lighthaving a second wavelength, which is directly formed on a portion of anupper surface of the substrate neighboring the first surface emittinglaser. The second surface emitting laser includes a second lowerreflector formed by alternately depositing two semiconductor materiallayers having different refractive indices and the same type ofimpurities. A second active layer is provided. A second upper reflectoris formed by depositing two semiconductor material layers havingdifferent refractive indices and the opposite type of impurities to thatof the second lower reflector. A lower electrode layer is formed on onesurface of the substrate, and first and second upper electrodes areformed on the first and second upper reflectors, respectively, forapplying electric power.

[0013] Also, to achieve another aspect of the above object, there isprovided a method for manufacturing a multi-wavelength surface emittinglaser comprising the steps of forming a first surface emitting laser foremitting light having a first wavelength by sequentially depositing on aprepared substrate a first lower reflector formed by alternatelydepositing two semiconductor material layers with the same type ofimpurities, but different refractive indices, a first active layer, anda first upper reflector formed by alternately depositing twosemiconductor material layers having the opposite type of impurities tothat of the first lower reflector and different refractive indices. Themethod also includes partially removing the first upper reflector, thefirst active layer, and the first lower reflection layer by etching, andforming a protection film on the outer surface of the first surfaceemitting laser. A second surface emitting laser for emitting lighthaving a second wavelength is formed by sequentially depositing on thesubstrate and the protection film, a second lower reflector formed byalternately depositing two semiconductor material layers having the sametype of impurities, but different refractive indices, a second activelayer, and a second upper reflector formed by alternately depositing twosemiconductor material layers having the opposite type of impurities tothat of the second lower reflector and different refractive indices. Themethod further includes removing the second lower reflector, the secondactive layer, and the second upper reflection layer formed on theprotection film by etching, and removing the protection film and formingfirst and second upper electrodes on upper surfaces of the first andsecond upper reflection layers, respectively, and a lower electrode on abottom surface of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above object and advantages of the present invention willbecome more apparent by describing in detail a preferred embodimentthereof with reference to the attached drawings in which:

[0015]FIG. 1 is a sectional view showing a conventional surface emittinglaser;

[0016]FIG. 2 is a sectional view showing a multi-wavelength surfaceemitting laser according to a preferred embodiment of the presentinvention; and

[0017]FIGS. 3A through 3F are sectional views showing the surfaceemitting laser at progressive stages leading to the completion of theinvention, for explaining a method for manufacturing themulti-wavelength surface emitting laser according to a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Referring to FIG. 2, a multi-wavelength surface emitting laser 20according to a preferred embodiment of the present invention includes asubstrate 30, and a first surface emitting laser 48 and a second surfaceemitting laser 49 provided on the substrate 30 for emitting laser beamshaving different wavelengths λ₁ and λ₂. Also, a lower electrode 35 isformed on the bottom surface of the substrate 30. A first upperelectrode 47 and a second upper electrode 57 are formed on the uppersurfaces of the first and second surface emitting lasers 48 and 49,respectively. Here, the first and second upper electrodes 47 and 57 havefirst and second windows 47 a and 57 a, respectively.

[0019] The substrate 30 is formed of a semiconductor material such asgallium arsenide (GaAs) or indium phosphorous (InP) including an n-typeimpurity.

[0020] The first surface emitting laser 48 emits a laser beam having thefirst wavelength λ₁ and includes a first lower reflector 41, a firstactive layer 43 and a first upper reflector 45, which are deposited inorder on the substrate 30. The first lower reflector 41 is a distributedBragg reflector (DBR) which is formed by alternately depositing pairs ofimpurity doped semiconductor layers having different refractive indices.The first lower reflector 41 is formed by alternately depositing n-typeAl_(x)Ga_(1−x)As and AlAs, which has the same impurity as the substrate30. The first upper reflector 45 is a DBR formed by alternatelydepositing pairs of impurity doped semiconductor materials havingdifferent refractive indices, and the opposite type of impurities tothat of the first lower reflector 41, for example, p-typeAl_(x)Ga_(1−x)As and AlAs.

[0021] The first active layer 43 has a single or multi-quantum wellstructure or a super lattice structure. The first active layer 43 is anarea in which, when current is applied to the first upper electrode 47and the lower electrode 35, holes and electrons generated by the firstupper reflector 45 and the first lower reflector 41, respectively arere-combined. The first active layer 43 emits a laser beam due to energytransition according to the combination of the electrons and the holes.To improve the output of light having the first wavelength λ₁ emittedthrough the first window 47 a, it is preferable to further provide afirst high resistance portion 46 by injecting protons into the firstupper reflector 45 except for a zone under the window 47 a. Thus, sincethe flow of current is limited by the first high resistance portion 46,light is emitted only from under the first window 47 a. Here, the firsthigh resistance portion 46 can be manufactured by an oxidation process,instead of the above-described proton injection method. The wavelengthof the light emitted from the first surface emitting laser 48 isdetermined by the structure of the first upper reflector 45 and thefirst lower reflector 41 and the structure of the first active layer 43.

[0022] The second surface emitting laser 49 emits a laser beam havingthe second wavelength λ₂ and includes a second lower reflector 51, asecond active layer 53, and a second upper reflector 55 which aredeposited in order on the substrate 30. The second lower reflector 51 isa distributed Bragg reflector (DBR) which is formed by alternatelydepositing pairs of impurity doped semiconductor layers having differentrefractive indices. The second upper reflector 55 is also a DBR formedby alternately depositing pairs of impurity doped semiconductor layershaving different refractive indices and an opposite semiconductor typeto that of the second lower reflector 51.

[0023] The second active layer 53 is an area where holes and electronsgenerated by the second upper reflector 55 and the second lowerreflector 51, respectivley, are re-combined when current is applied tothe second upper electrode 57 and the lower electrode 35. A laser beamis generated from the second active layer 53 due to energy transitionaccording to the combination of the electrons and holes. To improve theoutput of light of the second wavelength λ₂ emitted through the secondwindow 57 a, a second high resistance portion 56 is provided, which isformed by injecting protons into the second upper reflector 55, exceptfor a zone under the second window 57 a. Since the second highresistance portion 56 limits the flow of current, light is generated andemitted only from the zone under the second window 57 a. Here, thesecond high resistance portion 56 can be manufactured by an oxidationprocess instead of the above-described proton injection method. Thewavelength of the light emitted from the second surface emitting laser49 having the above structure is determined by the structure of thesecond upper reflector 55 and the second lower reflector 51 and thestructure of the second active layer 53. Also, the respective heights ofthe second upper reflector 55, the second lower reflector 51, and thesecond active layer 53 are set to be different from those of the firstsurface emitting laser 48.

[0024] The method for manufacturing the multi-wavelength surfaceemitting laser having the above structure will be described withreference to FIGS. 3A through 3F.

[0025] Referring to FIG. 3A, a substrate 30 is prepared and a firstsurface emitting laser 48 having a structure suitable for emitting lighthaving the first wavelength λ₁ is formed on the substrate 30. The firstsurface emitting laser 48 is formed by depositing the first lowerreflector 41, the first active layer 43, and the first upper reflector45 in order on the substrate 30. The first lower reflector 41 is formedby alternately depositing two semiconductor material layers with thesame type of impurities, but different refractive indices, while thefirst upper reflector 45 is formed by alternately depositing twosemiconductor material layers having different refractive indices andthe opposite type of impurities to that of the first lower reflector 41.Here, the first surface emitting laser 48 is manufactured by growing adesired semiconductor material on the substrate 30 by an epitaxialgrowth method using semiconductor equipment such as metal organicchemical vapor deposition (MOCVD) equipment or molecular beam epitaxial(MBE) equipment. Here, instead of the epitaxial growth method, the firstupper reflector 45 can be manufactured as a dielectric mirror formed ofa dielectric such as SiO₂ or TiO₂.

[0026] Next, part of the first surface emitting laser 48 is removed sothat part of the upper surface of the substrate 30 can be exposed. Asecond surface emitting laser 49 for emitting light having the secondwavelength λ₂ is formed in the area where part of the first surfaceemitting laser was removed. That is, referring to FIG. 3B, a firstetching mask 61 having a predetermined pattern is disposed on the firstupper reflector 45. Then, by performing etching, for example, by a dryetching method, parts of the first upper reflector 45, the first activelayer 43, and the first lower reflector 41 located in a predeterminedarea are removed.

[0027] Next, as shown in FIG. 3C, after the first etching mask 61 isremoved, a protection film 63 for enclosing the first surface emittinglaser 48 is formed. The protection film 63 is formed of silicon nitrideor silicon oxide. The protection film 63 is used to facilitate removalof an unnecessary portion growing on the upper surface of the firstsurface emitting laser 48 when a second surface emitting laser 49 isformed in a process which will be described later. The protection film63 also protects the first surface emitting laser 48 having completedits growth in a subsequent process.

[0028] Then, as shown in FIG. 3D, the second surface emitting laser 49is formed on the substrate 30 and the protection film 63. The secondsurface emitting laser 49 for emitting light having the secondwavelength λ₂ is formed by depositing the second lower reflector 51, thesecond active layer 53, and the second upper reflector 55 in order onthe substrate 30. The second lower reflector 51 is formed by alternatelydepositing two semiconductor material layers with the same type ofimpurities, but different refractive indices, while the second upperreflector 55 is formed by alternately depositing two semiconductormaterial layers having different refractive indices and the oppositetype of impurities to that of the second lower reflector 51. The secondsurface emitting laser 49 is preferably grown by the epitaxial growthmethod with the same semiconductor equipment used for the first surfaceemitting laser 48. Next, a second etching mask 67 is partially providedon the upper surface of the second upper reflector 55, as shown in FIG.3D, so that the remaining portion, except for part of the second surfaceemitting laser 49, formed neighboring the first surface emitting laser48, can be removed.

[0029] As shown in FIG. 3E, the remaining second surface emitting laser,except for an area under the second etching mask 67, is removed byetching. Then, the protection layer 63 is removed.

[0030] Finally, as shown in FIG. 3F, after the second etching mask 67 isremoved, the first and second upper electrodes 47 and 57, on which thefirst and second windows 47 a and 57 a for emitting light are provided,are formed on the upper surfaces of the first and second upperreflectors 45 and 55. The lower electrode 35 is formed on the bottomsurface of the substrate 30 and, thus, the multi-wavelength surfaceemitting laser 20 is completely manufactured.

[0031] Preferably, a step for partially forming the first and secondhigh resistance portions 46 and 56 by the proton injection process orthe oxidation process in each of the first and second upper reflectors45 and 55 is performed before the first and second upper electrodes 47and 57 are formed.

[0032] As described above, in the multi-wavelength surface emittinglaser manufactured according to the preferred embodiment of the presentinvention, since light having different wavelengths can besimultaneously emitted from a single substrate, optical alignment iseasy in comparison to a method in which surface emitting lasers,manufactured by an additional process, are attached to a substrate.Also, since the first and second surface emitting lasers aremanufactured in consecutive processes, the manufacturing process thereofis simple.

[0033] It is contemplated that numerous modifications may be made to theapparatus and method of the present invention without departing from thespirit and scope of the invention as defined in the claims.

What is claimed is:
 1. A multi-wavelength surface emitting laser foremitting light having a first wavelength and light having a secondwavelength, the laser comprising: a substrate; a first surface emittinglaser which emits light having a first wavelength, directly formed on aportion of an upper surface of the substrate and including, a firstlower reflector formed of two alternately deposited semiconductormaterial layers having a same type of impurity, but different refractiveindices; a first active layer; and a first upper reflector formed of twodeposited semiconductor material layers having different refractiveindices and an opposite type of impurity to that of the first lowerreflector; a second surface emitting laser which emits light having asecond wavelength, directly formed on a portion of an upper surface ofthe substrate neighboring the first surface emitting laser andincluding; a second lower reflector formed of two alternately depositedsemiconductor material layers having different refractive indices and asame type of impurity; a second active layer; and a second upperreflector formed of two deposited semiconductor material layers havingdifferent refractive indices and an opposite type of impurity to that ofthe second lower reflector; a lower electrode layer; a first upperelectrode formed on the first upper reflector, which electric power isapplied to; and a second upper electrode formed on the second upperreflector, which electric power is applied to.
 2. The laser as claimedin claim 1, further comprising a first high resistance portion partiallyformed in the first upper reflector, which limits a flow of current; anda second high resistance portion partially formed in the second upperreflector, which limits a flow of current.
 3. A method for manufacturinga multi-wavelength surface emitting laser comprising the steps of:forming a first surface emitting laser which emits light having a firstwavelength, the forming of the first surface emitting laser including,depositing, sequentially, on a prepared substrate a first lowerreflector, a first active layer and a first upper reflector, the firstlower reflector formed by; depositing, alternately, two semiconductormaterial layers having a same type of impurity, but different refractiveindices, the first upper reflector formed by; depositing, alternately,two semiconductor material layers having an opposite type of impurity tothat of the first lower reflector and having different refractiveindices; partially removing the first upper reflector, the first activelayer, and the first lower reflection layer by etching; and forming aprotection film on an outer surface of the first surface emitting laser;forming a second surface emitting laser which emits light having asecond wavelength, the forming of the second surface emitting laserincluding, depositing, sequentially, on the substrate and the protectionfilm, a second lower reflector, a second active layer, and a secondupper reflector, the second lower reflector formed by; depositing,alternately two semiconductor material layers having a same type ofimpurity, but different refractive indices, the second upper reflectorformed by; depositing, alternately, two semiconductor material layershaving an opposite type of impurity to that of the second lowerreflector and different refractive indices; removing portions of thesecond lower reflector, the second active layer, and the second upperreflection layer formed on the protection film by etching; and removingthe protection film; forming a first upper electrode on an upper surfaceof the first upper reflection layer; forming a second upper electrode onan upper surface of the second upper reflection layer; and forming alower electrode on a bottom surface of the substrate.
 4. The method asclaimed in claim 3, wherein the protection film is formed of oneselected from a group consisting of silicon nitride and silicon oxide.5. The method as claimed in claim 3, further comprising a step offorming first and second high resistance portions by injecting protonsinto part of the first and second upper reflectors before the first andsecond upper electrodes are formed.
 6. A multi-wavelength surfaceemitting laser for emitting light having a first wavelength and lighthaving a second wavelength, the laser comprising: a substrate; a firstsurface emitting laser which emits light having a first wavelengthformed on a portion of the substrate; and a second surface emittinglaser which emits light having a second wavelength formed on a portionof the substrate.
 7. The laser as claimed in claim 6, wherein the firstsurface emitting laser includes, a first lower reflector; a first activelayer; and a first upper reflector, and wherein the second surfaceemitting laser includes, a second lower reflector; a second activelayer; and a second upper reflector.
 8. The laser as claimed in claim 7,wherein the first lower reflector is formed of two alternately depositedsemiconductor material layers having a same type of impurity, butdifferent refractive indices, and the first upper reflector is formed oftwo deposited semiconductor material layers having different refractiveindices and an opposite type of impurity to that of the first lowerreflector, and wherein the second surface emitting laser is formed oftwo alternately deposited semiconductor material layers having differentrefractive indices and a same type of impurity, and the second upperreflector is formed of two deposited semiconductor material layershaving different refractive indices and an opposite type of impurity tothat of the second lower reflector.
 9. The laser as claimed in claim 7,further including, a lower electrode; a first upper electrode formed onthe first upper reflector, which electric power is applied to; and asecond upper electrode formed on the second upper reflector, whichelectric power is applied to.
 10. The laser as claimed in claim 6,wherein the first surface emitting laser and the second surface emittinglaser are formed on an upper surface of the substrate, and wherein thefirst surface emitting laser and the second surface emitting laserneighbor each other.
 11. A method for manufacturing a multi-wavelengthsurface emitting laser comprising the steps of: forming a first surfaceemitting laser which emits light having a first wavelength, on asubstrate; and forming a second surface emitting laser which emits lighthaving a second wavelength on the substrate.
 12. The method as claimedin claim 11, wherein the first surface emitting laser is formed by,depositing, sequentially, on the substrate a first lower reflector, afirst active layer and a first upper reflector, and wherein the secondsurface emitting laser is formed by; forming a protection film on anouter surface of the first surface emitting laser; and depositing,sequentially, on the substrate and the protection film, a second lowerreflector, a second active layer, and a second upper reflector.
 13. Themethod as claimed in claim 12, wherein the first lower reflector isformed by, depositing, alternately, two semiconductor material layershaving a same type of impurity, but different refractive indices, andwherein the first upper reflector is formed by; depositing, alternately,two semiconductor material layers having an opposite type of impurity tothat of the first lower reflector and having different refractiveindices, and wherein the second lower reflector is formed by;depositing, alternately two semiconductor material layers having a sametype of impurity, but different refractive indices, and wherein thesecond upper reflector formed by; depositing, alternately, twosemiconductor material layers having an opposite type of impurity tothat of the second lower reflector and different refractive indices. 14.The method as claimed in claim 12, further including, removing portionsof the second lower reflector, the second active layer, and the secondupper reflection layer formed on the protection film by etching; andremoving the protection film.
 15. The method as claimed in claim 14,further including; forming a first upper electrode on an upper surfaceof the first upper reflection layer; forming a second upper electrode onan upper surface of the second upper reflection layer; and forming alower electrode on a bottom surface of the substrate.
 16. Amulti-wavelength surface emitting laser for emitting light having afirst wavelength and light having a second wavelength, the lasercomprising: a substrate; first laser emitting means for emitting a laserhaving a first wavelength; and second laser emitting means for emittinga laser having a second wavelength.
 17. The laser as claimed in claim16, wherein the first surface emitting laser includes, first laserreflecting means; and first energy transition means for generating alaser beam, and wherein the second surface emitting laser includes,second laser reflecting means; and second energy transition means forgenerating a laser beam.
 18. The laser as claimed in claim 16, furtherincluding lower electrode means; first upper electrode means foraccepting electric power; and second upper electrode means for acceptingelectric power.